Frequency deviation in power generating systems has been a longstanding significant problem.
In the United States, electrical generating equipment is designed to output a 60 hertz wave (50 hertz elsewhere), and although the frequency is closely controlled, some deviation is inevitable as a result of sudden variations in the electrical load, lightning striking transmission equipment and a number of other factors. Frequency deviations in the output on the order of .+-.0.5 hertz are within acceptable limits, but any frequency deviation greater than 0.5 hertz, even those lasting only minutes, can cause significant damage to the generator, thus shortening the useful life of an asset often costing hundreds of thousands of dollars.
Several attempts have been made by others to solve this problem in order to prolong the life of the generating equipment. Most of these techniques exhibit a common deficiency in that they make simplifying assumptions about the output waveform which are incorrect, thus introducing their own error into the assumed correction. For example, many of these techniques incorrectly assume that the waveform to be analyzed is a pure sine wave, and that the time between zero crossings of the waveform may be considered an indication of the frequency from which the frequency deviation may be derived. While permitting the introduction of a correction factor, these assumptions introduce error of their own into the correction factor.
Other techniques convert the frequency variation into a proportional voltage output by using phase locked loops, the latter then being used to provide an output voltage based on the rate of change. This rate of change is obtained by using level detectors where the difference between the levels is used as indicative of rate of change of the frequency. In another technique, a few complete cycles of the output waveform are clocked and the output frequency corresponding to each cycle is calculated. Using this information, a curve fit technique is then employed to determine the rate of change of frequency. All of these techniques are deficient in that the accuracy of the estimated frequency deviation is affected by any non-60 hertz component that might be present in the voltage waveform.
In an attempt to overcome these deficiencies, techniques based on discrete Fourier transforms (DFT) have been developed. One such technique, termed the "leakage coefficient" technique, is based on the fact that all of the discrete Fourier transform components will be non zeros if any frequency deviation is present in the output waveform. However, prefiltering of the data is required before calculating the frequency deviation in order to avoid errors due to the presence of harmonics. The accuracy of DFT-based techniques may be affected by the presence of harmonics and/or the presence of white noise.
With the foregoing in mind, it is an object of the present invention to provide a technique for the estimation of frequency deviation which is not based on simplifying assumptions about the waveform to be measured.
Another object of the present invention is to provide a technique for the estimation of frequency deviation which is immune to errors induced by the presence of waveform harmonics or other noise signals.
A further object of the present invention is to provide a technique for the estimation of frequency deviation wherein prefiltering of the waveform output is not required.
A still further object of the invention is to provide a technique wherein measurement of the waveform output may be started at any point of the cycle of the periodic AC waveform.